A very limited amount of experimental work is reported in the prior art. A. Cibula (in an article entitled "The Mechanism of Grain Refinement of Sand Castings in Aluminium Alloys," written in the Journal of Institute of Metals, vol. 76, 1949, pp. 321-360) indicates that carbon in the master alloy does in fact influence grain refining. In the 1951-52 Journal of Institute of Metals, vol. 80, pp. 1-16, Cibula reported further work in the article, "The Grain Refinement of Aluminium Alloy Castings by Additions of Titanium and Boron". As indicated in the title, the effect of adding B and C to Al-Ti master alloys was studied. The results of this work on the effect of carbon is quoted directly from his paper:
"Although the results obtained above with titanium carbide additions conformed that it is possible to produce grain refinement with much smaller titanium additions than are normally used, no method of practical value was found. (Emphasis added.) The results showed that the obstacles in increasing the carbon content of aluminium [sic] titanium alloys are largely caused by the difficulty of achieving intimate contact and wetting between carbon or titanium carbide and molten aluminium, either due to interference by oxide films or to inherently unsuitable angles of wetting. It has been suggested that one way of avoiding the difficulty would be by pre-wetting titanium carbide powder by sintering with nickel or cobalt powder, but the high melting point of these metals would be inconvenient with aluminium alloys and bridging between carbide particles might prevent their complete dispersion." PA0 "The introduction of carbon into molten aluminium-titanium alloys is also limited by the low solubility of carbon in the melt, for any excess of carbide would tend to remain where it was formed, in contact with the source of carbon, instead of dispersing in the melt, unless the carbide could be precipitated in the liquid metal." PA0 "In the work described in the next section on the use of titanium boride instead of titanium carbide, the difficulties described above were overcome by using separate aluminium-titanium and aluminium-boron hardener alloys: by this means it was possible to precipitate the boride particles in the melt and control the excess of either constituent. This could not be done with titanium carbide additions because carbon cannot be alloyed with aluminium."
F. A. Crossley and L. F. Mondolfo wrote in the Journal of Metlas, 1951, vol. 3, pp. 1143-1148. In this report they found that the addition of A1.sub.4 C.sub.3, or graphite, to aluminum titanium melts resulted in a decrease in grain refining effect.
Further experiments in the art were recorded in 1968 by E. L. Glasson and E. F. Emley in an article in the book entitled "Solidification of Metals" (ISI publication No. 110, 1968), pp. 1-9. In this article, Glasson and Emley reported that C.sub.2 Cl.sub.6, or graphite, may be incorporated into salt tablets to improve grain refining by forming titanium carbide.
Further experiments in this area of research were reported by Y. Nakao, T. Kobayashi, and A. Okumura in the Japanese Journal of Light Metals, 1970, vol. 20, p. 163. Nakao and co-workers achieved essentially similar results by incorporating titanium carbide powder in a salt flux.
More recent experiments were reported in an article in the Journal of Crystal Growth, 1972, vol. 13, p. 777 by J. Cisse, G. F. Bolling, and H. W. Kerr. In this paper, the nucleation of aluminum grains was observed on massive titanium carbide crystals, and it was established that this epitaxial orientation relationship exists. EQU (001).sub.Al .vertline..vertline.(011).sub.TiC ; [001].sub.Al .vertline..vertline.[001].sub.TiC
More recently, A. Banerji and W. Reif briefly described an Al-6% Ti-1.2% C master alloy in Metallurgical Transactions, vol. 16A, 1985, pp. 2065-2068. This alloy was observed to grain refine 7075 alloy, and a patent application (No. 8505904 dated 3/1/85) was filed in the U.K.
A review of the prior art indicates that the problem has not been solved. Although there are indications that carbon may be beneficial in the grain refining of aluminum, massive carbides are found within the final product. This difficulty is summarized most succinctly in the second and third paragraphs of the above quotation from Cibula's 1951 study, and explains why boron, not carbon, has found commercial application as a third element in Al-Ti master alloys. Large, hard, insoluble particles cannot be present in master alloys used to refine alloys used in the manufacture of thin sheets, foil, or can stock. Large particles in thin products cause pinholes and tears.
This is essentially the crux of the problem: massive hard particles have prevented the development of an effective aluminum master alloy containing carbon. This invention has solved the problem.